In recent times much attention has been given to the effects of Zeta poential on the retention and formation of paper on the wire of a paper making machine. Zeta potential is an electrokinetic phenomenon associated with the charge developed on the particles that are suspended in the water of the feed stock or furnish and is affected by ion-producing chemicals commonly added to the paper for various purposes.
Zeta potential is usually measured in an indirect way by measuring a parameter dependent on it. One parameter used for the purpose is known as streaming potential, a potential difference which is established by the flow of liquid past stationary charged particles. A general discussion of Zeta potential and its effect is to be found in the paper "Electrokinetics in Papermaking--a Position Paper" by R. A. Stratton and J. W. Swanson in TAPPI, 64 No. 1, Page 79 (1981). Further discussion of Zeta potential together with a description of a system for measuring streaming potential is disclosed in the paper "Continuous Measurement of the Streaming Potential on a Paper Machine" by W. Sack of F. Schoeller, Jnr., in Das Papier 30, No. 10A, V42-V46 (1976).
What is clear from these papers is that while Zeta potential has an undoubted influence on paper formation, e.g. retention on the wire, flocculation and drainage, its effects cannot be quantified precisely. The optimum Zeta potential in a particular case depends on the paper to be made and hence the nature of the feedstock, including additives, and the characteristics of the paper-making machine in question. The work done so far, however, suggests that in most cases it is desirable to prevent excessive Zeta potential developing and that commonly it is required to maintain the Zeta potential at or near zero value for optimum results as regards the formation of paper on the wire.
Whatever the significance of Zeta potential and the mechanisms by which it influences paper formation in a particular case, there is a general need to be able to exercise control of Zeta potential in a paper-making process and consequently to be able to make a measure of it or of one of its dependent parameters such as streaming potential. The present invention is concerned with apparatus that is capable of making a measurement of streaming potential and with a control system including such measuring apparatus that is capable of maintaining the measured streaming potential at a set level which may be, but is not necessarily, zero.
The Sack paper above-mentioned describes an apparatus for measuring streaming potential in which a sample stream of white water--that is water drained from the wire--is passed through a horizontal cell in which the water is filtered by a vertical wire mesh on which a plug of the fibres and other particles in the water is built up. The streaming potential is developed across the plug and is measured by electrodes disposed in the cell on opposite sides of the plug (also referred to herein as a pad). The making of a further measurement requires rinsing of the cell to remove the plug and the establishment of a fresh plug.
While the Sack paper refers to the sample being white water or stock from the breastbox it appears from the paper that the measurements were all made on white water. It is generally desirable to provide apparatus that can be used reliably with the feed stock from the breastbox. The white water will contain a higher proportion of small fibres and fines since the larger fibres will be trapped on the wire.
There are factors which may, in our opinion, contribute to the Sack apparatus being unsatisfactory for practical use. In the horizontal cell shown there will be a tendency to sedimentation as the plug builds up leading to non-uniform plug, that is a wedge-shaped plug thicker at the lower part of the cell. This tendency would be even greater in measurements made on the feed stock than on the white water. In addition proper rinsing or flushing of the cell may be difficult to achieve consistently in the horizontal cell. The plug or pad is compacted to a degree that causes it to be released as a substantially unitary body. This body must be removed as completely as possible together with any remaining particles on the filter structure. Furthermore, the streaming potential measurement as made by Sack and which is dependent on the pressure difference across the plug, is made during build-up of the plug. The potential is taken as the difference between the potentials measured as a low pressure limit is exceeded as the plug starts to build up and as a high pressure limit is reached when the plug is far more fully formed. Consequently, the measurement is not made with the same plug. It is not necessary here to enter into a full discussion of streaming potential and the factors that affect it. Reference may be made to "Colloid Science"--Vols. I and II; Editor: Kruyt; published by Elsevier.
There will be described hereinafter apparatus in accord with the present invention which is intended to at least mitigate the disadvantages set forth above so as to lead to measurements of sufficient reliability to be used in the control of a paper-making machine.